1. Field of the Invention
This invention relates to corrosion measurement, and more particularly to methods for placing a corrosion specimen into an environment to be tested.
2. Description of the Prior Art
It has long been the practice in the monitoring of the corrosion rates of metals exposed to a corosive environment to place corrosion test specimens, such as probes or coupons, in the corrosive environment. The corrosion rate of the metal in the particular environment is determined by the change in weight of these specimens as a function of the exposure time. Additionally, visual inspection of the specimens can reveal the type of corrosion which has occurred, such a a uniform general corrosion or a more localized attack such as pitting.
The measured corrosion rate and the determined type of corrosion are subsequently used as the base for selection of a corrosion protection system. Similarly, the specimen can be used to monitor the effectiveness of corrosion protection systems. Often, specimens of proposed construction materials for a structure are exposed to the expected environment in order to determine their susceptibility to corrosion.
In many cases, the environment of interest is not directly accessible and the specimen must be passed through one or more contaminating environments. Exposure to a corrosive or erosive gas can accelerate the subsequent corrosion rate. Also, exposure to a liquid hydrocarbon or other film-forming compositions can leave a corrosion-inhibiting film. In these cases, the measured corrosion rate may not be representative of the actual corrosion rate and may not represent the corrosion type.
In particular, the measurement of corrosion in the bottom of oil storage tanks, in which a liquid hydrocarbon layer floats on an underlying aqueous corrosive layer, presents a problem. A corrosion test specimen which is to be placed in the aqueous layer must be passed through the upper hydrocarbon layer. The specimen will usually acquire a hydrocarbon film which will inhibit corrosion until the film has been dissipated. The dissipation time is dependent on the hydrocarbon type and the texture and uniformity of the surface of the corrosion specimen. In the case where the hydrocarbon film is uneven or broken, corrosion of the specimen would vary across the surface and give the appearance of localized attack even if there would be little localized attack without the hydrocarbon film. Since the choice of a corrosion protection system depends on the type as well as the rate of corrosion, examination of an improperly exposed specimen could result in the selection of an improper or inadequate system. The results of such selection could be unchecked corrosion or, conversely, expensive over-protection. In a few instances, such as when chemical corrosion inhibitors are used to provide an anodic coating, an improper choice of inhibitor concentration or type can actually convert a minor, general corrosion problem into a severe localized corrosion problem.
Thus, need exists for a method for protecting the prepared surface of a corrosion specimen from a contaminating environment while the specimen is being placed in the environment to be tested.
Accordingly, a principal object of this invention is to provide a method for monitoring corrosion.
Yet another object of this invention is to provide an improved method for placement of corrosion test specimens into an environment to be tested.
Still another object of this invention is to provide a method for protecting the prepared surface of corrosion test specimens from contamination by a hostile environment during their placement into the environment of interest.
A further object of this invention is to provide a method for protecting the prepared surface of a corrosion test specimen from acquiring a hydrocarbon film while the specimen is passed through the hydrocarbon layer of an oil storage tank and into the aqueous layer to be tested.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description.